Chemistry: Principles and Reactions (Harcourt)
Ch 6 p. 174 #2(a,b) & #8
2)
A photon of light has a wavelength of 423 nm. Calculate
a)
the frequency
c
 

3.00  10 8 ms
 
423  10 9 m
  7.09  1014 Hz
b)
the energy in joules per photon
E = h
E = (6.626 x 10-34 J·s)(7.09 x 1014 s-1)
E = 4.70 x 10-19 J
8) Microwave ovens heat food by the energy given off by microwaves. These microwaves have a
wavelength of 5.00 x 106 nm.
a)
How much energy in kilojoules per mole is given off by the microwave oven?
See p. 147 for an appropriate example to follow.
E
E
hc

(6.626  10 34 J  s )(3.00  10 8 m s )
5,000,000  10 9 m
E  3.9756  10 23 J
E  3.9756  10 23 J 
b)
1kJ
6.02 x10 23

 0.0239 kJ mol
1000J
1mol
Compare the energy obtained in (a) with that given off by the ultraviolet rays (≈100 nm)
of the sun.
hc
E

(6.626  10 34 J  s )(3.00  10 8 m s )
100  10 9 m
E  1.9878  10 18 J
E
E  1.9878  10
18
1kJ
6.02 x10 23
J

 1197 kJ mol  1000 kJ mol
1000J
1mol
The rays from the sun give off much more energy per mole than microwaves.